Process for carbonizing fibrous cellulosic materials



1967 P. L. M WHORTER ETAL 3,337,301

PROCESS FOR CARBONIZING FIBROUS CELLULOSIC MATERIALS FilGd Jan. 17, 196

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ATTORNEYS United States Patent ()fiice 3,337,301 Patented Aug. 22, 19673,337,301 PRGCESS FOR CARBONIZING FIBROUS CELLULQSIC MATERIALS Purnal L.McWhorter, Odessa, and Ernest 0. hsol, Wilmington, Del, assignors toHavey Industries, Inc, a

wholly owned subsidiary of Hercules Powder Company,

New Castle, Del., a corporation of Delaware Filed Jan. 17, 1964, Ser.No. 338,492 10 Claims. (Cl. 23-2091 ABSTRACT OF THE DISCLOSURE A processfor the preparation of carbon fibers by pyrolyzing cellulosic fibers,preferably in fabric form, in a non-oxidizing gaseous atmospherecontaining a predominant amount of steam.

This invention relates to the preparation of carbon fibers.

Previous proposals for making carbon fibers from cellulosic materialsfrequently have suffered from the disadvantage of requiring relativelyexpensive types of inert atmosphere and/ or difiiculties in controllingthe rate of carbonization to prevent too rapid a devolatilization duringthe early critical stages.

It is an object of the present invention to prepare good quality carbonfibers from regenerated cellulose, e.g. in the form of yarn, clothindividual fibers or the like, with reproducible results.

Another object is to devise an improved procedure for forming carbonfibers from regenerated cellulose.

An additional object is to employ an economical atmosphere in formingcarbon from cellulose fibers.

A further object is to moderate the rate of carbonization by preventingtoo rapid a devolitalization in converting cellulosic fibers to carbon.

Yet another object is to provide a desirable reducing atmosphere duringcarbonization.

A mere specific object is to prepare improved carbon fibers, e.g. in theform of cloth, from viscose rayon.

Still further objects and the entire scope of applicability of thepresent invention will become apparent from the detailed descriptiongiven hereinafter; it should be understoocl, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

It has now been found that these objects can be attained by pyrolyzingcellulosic fibers, preferably in fabric form, to carbon using steam assuch or steam which has been partially decomposed over hot carbon as thepurging medium.

The present invention is suitable for conversion of various forms ofcellulosic fibers into carbon fibers. Thus it can be employed withcotton fibers, cotton linters, cotton cloth, flax or other cellulosicfiber-s. Preferably, however, regenerated cellulose fibers are employed,most preferably in the form of cloth. As used in the presentspecification and claims the term regenerated cellulose is intended toembrace fibers of viscose rayon, cuprammonium rayon and saponifiedcellulose ester rayon, e.g. saponified cellulose acetate rayon.

The regenerated cellulose can be employed in the form of cord, yarn,cloth or the like, but preferably is in the form of cloth.

The invention will be best understood with reference to the accompanyingdrawing which is a diagrammatic illustration of one form of theinvention.

The carbonization is carried out in a non-oxidizing atmosphere.

Steam can be used by itself or it can be diluted with an inert gas suchas nitrogen, helium or argon or it can be diluted with a reducing gassuch as hydrogen, deuterium or carbon monoxide when employed inpyrolyzing the cellulose in the temperature range of 200 to 700 F.,preferably not over about 600 F. The steam is 20 to 100% of the gas bymol percent.

When the steam is employed in pyrolyzing at temperatures above 700 F.,it is first passed over a bed of reactive carbon 'to prevent loss by thewater gas reaction of the carbon fibers in the cloth being carbonized.Preferably the bed of reactive carbon is maintained at a temperatureequal to or above the temperature of the cloth being carbonized. Ifdesired it is also possible to use steam which has thus been partiallybroken downinto carbon monoxide and hydrogen in the earlier and criticalpyrolysis of the cellulosic fibers at temperatures of 700 F. or below.

If desired small amounts of additional reagents can be introduced withthe steam. Thus for example, steam volatile acid catalysts such asacetic acid, hydrochloric acid, formic acid, oxalic acid, propionicacid, malonic acid can be introduced into the liquid water beingvaporized to generate the steam which is to be passed through thecarbonization chamber. These acids can be added to the water so thatthere is 0.05 to 2 mol percent of acid based on the volume of steambeing passed into the carbonization chamber. Such acids help toneutralize any alkali which may still be present in the cellulose, e.g.regenerated cellulose.

The ccllulosic material is purified in conventional fashion prior toinitiation of the carbonization treatment. Thus there can be usedcommercial viscose or other re generated cellulose which has a sodiumcontent below 25 ppm. or if the cellulose has above 25 ppm. of sodium itcan be reduced below 25 ppm. by washing with water or dilute acid. Therecan also be used cellulosic fibers having a sodium content above 25 ppm.but for most consistent results the sodium content should be below Thecellulose, e.g. regenerated cellulose fabric is heated in the atmosphereof steam, with or Without non-oxidizing diluting gases as indicatedabove, to a temperature of 200 to 400 F. The heating to the upper end ofthis range can be either rapid or slow and the time is not critical. Itis critical, however, that the heating in the temperature range of 400to 550 F. be quite slow. Thus the heating up to 380 F. can beaccomplished in 2 minutes but the heating in the range of 300 to 550 F.,most preferably 400 to 550 F, should be for at least one-half hour andpreferably is at least 2 hours and can be much longer, e.g. 12 hours oreven a week. The product is then gradually heated to a temperature offrom 550 to 800 F. for 1 /2 to 24 hours or longer, usually 4 to 12hours, next the temperature is raised from 800 to 1500 F. or above overa period of -2 to 18 hours. At a top temperature of 1500 F. the producthas 94% carbon, at a top temperature of 1800 F., the product has 96.5 to97.5% carbon, and at a top temperature of 2000 F. the product contains98 to 99% carbon. If the final temperature is elevated to 3500 F. orabove, e.g. 5000 F., the product is converted into graphite fibers orcloth.

It has been found desirable to use a final temperature of 1800 F.because the resulting fiber product, whether in the form of cord, yarnor cloth is very strong.

A preferred heating schedule comprises a slow heating rate of 18 F. toF. rise per hour from a temperature of 212 F. to 750 F. followed by aheating rate of up to 215 F. per hour to a temperature of 1650 F. fol

lowed by further heating at a rate up to 5400 F. per hour until thedesired maximum temperature is reached.

It has been found that the steam is considerably more economical to usethan other gaseous media for the pyrolysis. Steam also has a moderatingefiect on the rate of carbonization preventing too rapid adevolatilization during the critical heating stages. Steam also hasdesirable heat capacity and heat transfer characteristics. When steam ispartially decomposed in a water gas reaction ahead of the carbonizationchamber, a desirable reducing atmosphere is provided.

Unless otherwise indicated all percentages are mol percent.

Example 1 Painesville White heavy duty viscose rayon cloth was heatedfrom about 200 F. to 550 F. in the presence of a current of steam. (ThePainesville White rayon had 3400 filaments in the yarn and 20 x 20threads per square inch). The heating to a temperature of about 400 F.was quite rapid. The rate of heating was controlled to provide a uniformtemperature in the cloth and to avoid unduly rapid generation ofvolatiles in the pyrolysis temperature range of 400 to 550 F. A total of3 hours was required to pass through the 400 F. to 550 F. temperaturerange.

The thus pyrolyzed cloth was ready for further heating in conventionalfashion, e.g. with an inert gas such as nitrogen, argon, helium or usinga vacuum, e.g. .001 mm. or employing a reducing gas such as hydrogen orcarbon monoxide. Thus in one example the cloth was placed in a nitrogenatmosphere and heated from 550 F to 800 F. over a period of 6 hours andfinally heated at a temperature increasing from 800 to 1800 F., over 6hours to obtain a carbon cloth having a carbon content I over 97%.

Example 2 Viscose rayon cloth having 3400 filaments in the yarn andhaving 20 x 20 threads per square inch and a sodium content of 7 ppm.was heated from about 200 to 550 F. in a current of steam and nitrogencontaining 80 mol percent steam and the balance being nitrogen. The rateof heating was such that it took 4 hours to pass through the 400 F. to550 F. temperature range. The temperature of the cloth was raised to atemperature of 400 F. in about 5 minutes.

The thus pyrolyzed cloth was then heated from 5 50 F. to 800 F., over aperiod of 5 hours and finally heated at a temperature increasing from800 to 1800 F. over 6 /2 hours to obtain carbon cloth having a carboncontent over 97%.

Example 4 Painesville White viscose rayon cloth was heated from 212 F.to 550 F. in the presence of a current of steam. The heating to atemperature of 380 F. was quite rapid (less than 5 minutes), the rate ofheating was then controlled so that the temperature rose from 380 F. to550 F. over a period of 4 hours. The thus pyrolyzed cloth was thenheated gradually from 550 F. to 1800 F. over a period of 12 hours in thepresence of steam which had previously been passed over a bed ofreactive carbon particles. The steam was passed over the carbon bed at atemperature 100 F. above that at which it was used as an atmosphere forthe heating of the partially pyrolyzed cloth and then was cooled to theindicated pyrolyzing temperatures. Thus as the pyrolyzing temperaturewas gradually increased from 550 F. to 1800 F. the steam employed Waspassed over the carbon bed 4 at a temperature gradually increased from650 to 1900 F.

Example 5 The procedure of Example 4 was repeated except that the steamemployed in heating the cloth from 212 to 550 F. was steam that had beenpassed over a bed of reactive carbon particles at a temperature whichwas gradually increased from 300 to 650 F. The steam was cooled to therequisite temperature of 212 to 550 F. prior to introduction into thechamber containing the cloth to be pyrolyzed.

Example 6 The procedure of Example 4 was repeated except that hydrogenchloride vapor in an amount of 0.2 mol percent of the steam wasintroduced into the steam prior to passage through the carbonizationchamber at 212 to 550 F.

It is also possible to add a minor amount of nitrogen, argon, helium orother nonoxidizing gas to the steam partially decomposed by the watergas reaction prior to introducing the thus decomposed steam to the clothor other fiber to be carbonized.

What is claimed is:

1. In a process of preparing carbon fibers from cellulosic fibers theimprovement comprising carrying out the pyrolysis of partiallycarbonized cellulosic fibers at a temperature of above 700 F. in thepresence of steam which has carbon monoxide and hydrogen therein as aresult of having been passed over reactive carbon at a temperature atleast equal to the pyrolysis temperature prior to treatment of thepartially carbonized fibers therewith.

2. A process according to claim 1 wherein the cellulosic fibers areregenerated cellulose fibers.

3. A process according to claim 2 wherein the atmosphere consists of atleast 20 mol percent of steam and the balance other nonoxidizing gases.

4. A process according to claim 3 wherein the nonoxidizing gases areinert gases.

5. A process according to claim 2 wherein the regenerated cellulosefibers are in the form of cloth.

6. In a process of preparing carbon fibers from cellulosic fibers thesteps comprising carrying out the pyrolysis of the cellulosic fibers inthe range of 400 to 700 F. in a nonoxidizing atmosphere comprising apredominant amount of steam to form partially carbonized fibers andcarrying out the pyrolysis of the partially carbonized fibers at atemperature above 700 F. in a nonoxidizing atmosphere which ispredominantly a mixture of steam, carbon monoxide and hydrogen resultingfrom the partial decomposition of the steam by passing the same overreactive carbon at a temperature at least equal to the pyrolysistemperature.

7. A process according to claim 6 wherein the cellulosic fibers are inthe form of regenerated cellulose cloth.

8. A process according to claim 1 including a small amount of a volatilenonoxidizing acid in the steam.

9. A process according to claim 1 wherein the cellulosic fibers areviscose rayon cloth.

10. A process according to claim 6 wherein the cellulosic fibers areviscose rayon cloth.

References Cited UNITED STATES PATENTS 3,011,981 12/1961 Soltes 252-5023,053,775 9/1962 Abbott 252-421 3,107,152 10/1963 Ford et al. 23209.23,116,975 1/1964 Cross et al. 23209.4 3,179,605 4/1965 Ohsol 252-502OSCAR R. VERTIZ, Primary Examiner.

EDWARD J. MEROS, Examiner.

1. IN A PROCESS OF PREPARING CARBON FIBERS FROM CELLULOSIC FIBERS THEIMPROVEMENT COMPRISING CARRYING OUT THE PYROLSIS OF PARTIALLY CARBONIZEDCELLULOSIC FIBERS AT A TEMPERATURE OF ABOVE 700*F. IN THE PRESENCE OFSTEAM WHICH HAS CARBON MONOXIDE AND HYDROGEN THEREIN AS A RESULT OFHAVING BEEN PASSED OVER REACTIVE CARBON AT A TEMPERATURE AT LEAST EQUALTO THE PYROLYSIS TEMPERATURE PRIOR TO TREATMENT OF THE PARTIALLYCARBONIZED FIBERS THEREWITH.